I have an opportunity to get a new 2.5" catalytic converter from a friend for a great price.

Is there any harm to using a 2.5" cat on my 4-banger duke instead of the stock size which is probably 2'' or so?

Yes, I need a new cat as my excellent running '84 duke failed the idle HC part of the local smog test while running a gutted cat.

stock is 2.5 inch i believe...just swapped in some new stuff myself.

quote Originally posted by Hockaday: stock is 2.5 inch i believe...

Well, that's interesting. I swear I read the exhaust system on a duke (if not both the duke and the V6) was more like 1 7/8" to 2".

We need some more feedback here folks!

Stock is not larger than 2".

Having said that...
My 4.9 has a 2.5" system adapted up to a 3" cat. Works just fine.
I say go for it.

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Raydar
88 4.9 Formula IMSA Fasback..........................88 3.4 coupe - going...going... gone to a new home.

Read Nealz Nuze! Praise the Lowered!

Alright, sounds like there should be no major issues.

Thanks for the feedback, guys!

I can't see there being any issues either.

Devils advocate with the bucket of cold water (maybe), here goes...

First thing that came to mind was that the Duke has an EGR valve that operates partially off of exhaust back pressure. Larger catalytic might equal less back pressure which would have the effect of less EGR.

While less EGR usually means more "performance" as reported by forum members who have blocked or disabled their EGR on the Duke, less EGR results in higher combustion temperatures and that affects the emission readings. I guess if someone was installing a no cat, free flowing exhaust it might be an issue for the Duke EGR but this is so close to stock, shouldn't matter.

I honestly don't think it'll make that much difference at all but it would be interesting to have the scientific data on both measured cases. I firmly believe that you'll pass emissions testing with flying colors, just would like to see the readings.

I'm not the emissions expert by any means, please don't flame me, just something that popped into my mind, this intrigues me somewhat, that's all.

The only problem you may encounter is it will take longer to get to operating temp, which may fail you if it is not. That's talking more about the physical size of the catalyst then the pipe diameter. If the catalyst is the same size, or smaller then the original one, and the pipe diameter is just larger, you should be fine. Back pressure is determined by the greatest restriction the the system, having a larger cat with the same head pipe and tail section opening won't change that any unless the cat was the greatest restriction to begin with. Also do not put the catalyst any father down stream then it was. In fact if you can move it anymore towards the Y pipe it will get up to temp quicker and be more effective.

..The system is design to have a certain neck down to improve scavenge from exhaust valve ,this only works at higher RPM..There should be no problem,,the stock pipe should be 2 inch..
..just make sure the engine is HOT if you have mandatory emission inspection,when you go thru check

Thanks for all the extra feedback, guys.

This little 5-speed '84 duke goes pretty good with the gutted cat and a GT (dual) exhaust. I've autocrossed it a couple times now and I've kept up with the "stock" V6 Fieros that were also there.

My biggest complaint with the gutted cat is that with it the exhaust sounds like crap. It sounded much better (deeper) before the cat plugged up and I had to disembowel it. Now it just sounds plain noisy.

So the fact that the duke failed the smog test is not such a bad thing as it's forcing me to replace the cat. Soon, the little '84 will be sounding sweet (in a gear-head kind of way) again.